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Trimeresurus
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Question about autopilot/autothrottle disengagement

Sun Sep 06, 2020 9:55 pm

Why does moving the thrust levers while autothrottle is engaged doesn't disengage it but moving the yoke(beyond a certain tiny point) while autopilot is engaged disengages it?(737NG, but I assume other Boeing and McDonnell/Douglas planes have the same philosophy)
 
GalaxyFlyer
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Re: Question about autopilot/autothrottle disengagement

Mon Sep 07, 2020 1:01 am

I don’t have an explanation, but it’s bad technique in any case.
 
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Starlionblue
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Re: Question about autopilot/autothrottle disengagement

Mon Sep 07, 2020 3:18 am

At a guess, being able to rapidly disengage the autopilot when pulling can be useful in a situation where stress leads the brain forgetting stuff. Say the autopilot is about to fly you into a mountain.

For the thrust levers, the same level of urgency may not be required. Also, and correct me if I am wrong, on Boeing if you move the thrust levers this overrides the autothrottle setting anyway until you let go again.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
TonyClifton
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Re: Question about autopilot/autothrottle disengagement

Mon Sep 07, 2020 3:36 am

Starlionblue wrote:
At a guess, being able to rapidly disengage the autopilot when pulling can be useful in a situation where stress leads the brain forgetting stuff. Say the autopilot is about to fly you into a mountain.

For the thrust levers, the same level of urgency may not be required. Also, and correct me if I am wrong, on Boeing if you move the thrust levers this overrides the autothrottle setting anyway until you let go again.

Same on the ERJ-170/190. The A/T system is very sluggish, so you’ll end up overriding on approach on any day without calm winds. I called it “student pilot mode” when it would be bouncing between +/- 10 REF.
 
Max Q
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Re: Question about autopilot/autothrottle disengagement

Mon Sep 07, 2020 5:05 pm

Starlionblue wrote:
At a guess, being able to rapidly disengage the autopilot when pulling can be useful in a situation where stress leads the brain forgetting stuff. Say the autopilot is about to fly you into a mountain.

For the thrust levers, the same level of urgency may not be required. Also, and correct me if I am wrong, on Boeing if you move the thrust levers this overrides the autothrottle setting anyway until you let go again.



Correct


It’s not good technique to make manual inputs to the AP or AT when they’re engaged anyway, instead of attempting to override them they should be disconnected if one or both is not doing what you want it to do


The 757 AT could be ‘lazy’ at high altitude when you were close to your weight limits
and would occasionally reduce thrust too much resulting in speed getting too slow, by the time it would correct itself even full power would not recover the lost airspeed



In that case you might override the AT if it attempted to reduce power excessively, it also didn’t always cope well with mountain wave activity, not reacting quickly enough to airspeed changes, once again you might override them in that case but carefully


You need to understand why they are making those inputs and not override one that’s necessary!
The best contribution to safety is a competent Pilot.


GGg
 
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fr8mech
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Re: Question about autopilot/autothrottle disengagement

Mon Sep 07, 2020 8:26 pm

Some time ago I was jumpseating on an MD11 and while descending, all the way through the approach, the captain would continue to bring the throttles to idle, or near enough. While on final, he mentioned to the FO that “he hates when it does that”.

Being in maintenance, after the flight I asked him if it was a problem with the aircraft. His reply was “nope, I just don’t like the way the airplane flies the throttle.”
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Max Q
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Re: Question about autopilot/autothrottle disengagement

Mon Sep 07, 2020 9:23 pm

fr8mech wrote:
Some time ago I was jumpseating on an MD11 and while descending, all the way through the approach, the captain would continue to bring the throttles to idle, or near enough. While on final, he mentioned to the FO that “he hates when it does that”.

Being in maintenance, after the flight I asked him if it was a problem with the aircraft. His reply was “nope, I just don’t like the way the airplane flies the throttle.”



Not familiar with the MD11 auto flight system but in certain descent modes the 757/ 67 AT will not reduce thrust all the way to idle


VS is rarely used but it will not go all the way to idle and FLCH may not if the altitude change is minimal (forget the exact numbers)


So pulling them back to idle manually from slightly above is not uncommon
The best contribution to safety is a competent Pilot.


GGg
 
CosmicCruiser
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Re: Question about autopilot/autothrottle disengagement

Tue Sep 08, 2020 12:10 am

fr8mech wrote:
Some time ago I was jumpseating on an MD11 and while descending, all the way through the approach, the captain would continue to bring the throttles to idle, or near enough. While on final, he mentioned to the FO that “he hates when it does that”.

Being in maintenance, after the flight I asked him if it was a problem with the aircraft. His reply was “nope, I just don’t like the way the airplane flies the throttle.”

I never saw a problem with the MD-11. The MD-10 sucked on landing and the flair. I always disconnect the A/T on landing. I would always disconnected the throttles on gusty x-winds on both.

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